Classification of laser self-mixing interferometric signal under moderate feedback

This paper presents a different approach to classify self-mixing (SM) signals operating in the moderate feedback regime. A total of six distinct classes of SM signals can be defined based on the SM inherent shapes, which depend on both the feedback factor C and the linewidth enhancement factor α. This classification allows clear identification of SM signals for which normalization issues can arise and thus for which displacement precision is inherently reduced due to the very nature of the signal itself. Finally, it is shown that phase unwrapping approaches can theoretically retrieve displacement with subnanometer precision for usual laser diodes in the moderate feedback regime, in the absence of noise, only for α values greater than approximately 4.3.